This invention relates to variable area exhaust nozzles for gas turbine engines and, more particularly, to variable area exhaust nozzles which act as air inlets when the engine is operated in a reverse thrust mode.
The exhaust nozzle of a gas turbine engine, such as a turbojet or turbofan engine, has as its purpose the transformation of the pressure and thermal energy of the combustion discharge or exhaust gases into velocity, the forward thrust of the engine being directly proportional to the increase in velocity of the gas from the entrance of the engine to the exit plane of the nozzle. It has been found desirable to permit variation of nozzle flow area to maintain a high engine performance under a wide range of operating conditions. For example, it may be desirable to provide a larger nozzle flow area during a take-off mode of operation than during a cruise mode. One means for varying the nozzle flow area is by the use of a plurality of movable members or flaps pivotally supported about the nozzle axis. One of the problems associated with use of a plurality of flaps concentrically arranged about the engine axis has been the need to maintain effective seals between the flaps as the flaps are adjusted to vary the nozzle flow area so as to optimize the nozzle performance throughout various modes of engine operation. Therefore, it is desirable to provide a smooth transition or seal between the side edges of the flaps making the exhaust nozzle aerodynamic in shape and efficient in its operation.
Thrust reversers are devices which redirect the exhaust gas flow from the gas turbine engine in a direction opposed to that of the exhaust gas flow during normal operation. Reversible variable pitch fan jet engines operate in a thrust reversal mode by reversing the flow of air through a fan duct so that the engine inlet becomes the exhaust duct and the fan exhaust duct becomes the engine inlet. Since fan exhaust nozzles are normally sized to accelerate the gas flow to high velocities prior to exiting, when used as an inlet during a reverse thrust mode of operation, the flow area of the nozzle is too small to allow the required large volumes of air to enter at the required low velocity flow rate. Therefore, it is desirable to provide exhaust nozzle flaps which may be opened wider than normal to define a nozzle flow area of sufficient magnitude to enable the nozzle to effectively operate as an inlet in a reverse thrust mode operation. However, the actuation or positioning of flaps of sufficient size may require a large number of actuators or actuators of high power ratings unless a sufficient mechanical advantage is provided for between the actuators and the flap mounting means.
It is therefore an object of this invention to provide an improved variable area exhaust nozzle for a gas turbine engine wherein adjacent flaps employed therein are maintained in a sealed arrangement making the nozzle aerodynamic in shape and efficient in its operation.
Another object of this invention is to provide an improved variable area exhaust nozzle wherein a mechanical advantage between the actuators and the hinges is achieved when the nozzle flaps are adjusted.